1. Field of the Invention
The present invention relates to a radiographic image detecting apparatus for detecting radiation that has passed through a human body, and a radiographic image capturing system which incorporates such a radiographic image detecting apparatus therein.
2. Description of the Related Art
In the medical field, there have been employed portable radiographic image detecting apparatus such as an FPD (Flat
Panel Detector) or the like for detecting the intensity of radiation that has passed through a human body thereby to capture images of regions in the human body. The FPD, which is also referred to as “electronic cassette”, is capable of capturing an image of a region in a patient while the patient is lying on the bed or the like. The FPD is also capable of adjusting a region to be imaged by changing its own position. Consequently, the FPD can be flexibly used for image patients who are unable to move their own bodies.
Under such circumstances, various radiographic image detecting apparatus have been proposed in the art. One such radiographic image detecting apparatus is of the direct conversion in which photoelectric conversion elements generates electric charges in response to radiation applied thereto and then converts the generated electric charges into electric signals. Another radiographic image detecting apparatus is of the indirect conversion in which a scintillator or the like converts radiation applied thereto into an electromagnetic wave such as visible light or the like having a wavelength other than the wavelength of the radiation, and photoelectric conversion elements such as photodiodes or the like generate electric charges in response to the electromagnetic wave applied thereto and converts the generated electric charges into electric signals.
In such radiographic image detecting apparatus, the photoelectric conversion elements are disposed in respective areas delimited by a plurality of scanning lines and a plurality of signal lines that are disposed on a substrate such as a glass substrate, a flexible substrate, or the like so as to intersect one another. Radiation or an electromagnetic wave converted from radiation is applied to the photoelectric conversion elements, whereby the photoelectric conversion elements generate and store electric charges. The stored electric charges are read from the photoelectric conversion elements, i.e., pixels, through the signal lines as electric signals.
The radiographic image detecting apparatus in the form of FPDs are, however, problematic in that if a wide dynamic range for covering various radiation doses ranging from a low radiation dose to a high radiation dose is to be maintained, then since components of amplifying circuits and other electric circuits for processing electric signals tend to generate noise such as thermal noise, shot noise, or the like, the SN ratio is lowered particularly in a low radiation dose range, resulting in a reduction of the quality of produced radiographic images.
Japanese Laid-Open Patent Publication No. 2009-219538 (hereinafter referred to as JP2009-219538A) discloses a technology for solving the above problem. According to the disclosed technology, attention has been focused on the fact that the amounts of bias currents flowing through bias lines from the photoelectric conversion elements in order to bias the photoelectric conversion elements vary depending on the dose of radiation applied to the photoelectric conversion elements. Based on the fact, the dose of radiation applied to the photoelectric conversion elements is calculated, and the gain of an amplifying circuit at the time electric signals are read from the photoelectric conversion elements is established based on the calculated dose of the radiation.
Japanese Laid-Open Patent Publication No. 2010-212925 (hereinafter referred to as JP2010-212925A) discloses a radiographic image capturing system wherein a detector includes a plurality of areas, and current detectors according to JP2009-219538A for detecting currents flowing through the bias lines for the respective areas, and no image data are read from those areas that do not detect an applied radiation, for thereby shortening the time required to send image data.
According to JP2009-219538A and JP2010-212925A, for calculating the amounts of bias currents, the bias lines through which the bias currents for the respective photoelectric conversion elements flow are converged on a line, and a resistor having a large resistance value of 100 [kΩ] or 1 [MΩ] is connected in series to the line. Then, a voltage generated across the series-connected resistor is measured by a differential amplifier, because the bias currents are weak, i.e., in the order of microampere.